The invention relates to a wake-up circuit for an electronic device comprising an input circuit for receiving infrared signals and an amplifier connected to the input circuit, the output of the amplifier being connected to a switching circuit for switching on the device.
Such circuits, termed xe2x80x9cwake-upxe2x80x9d circuits, are utilized e.g. in TV sets or generally in receivers to switch on a device which is in the xe2x80x9cstand-byxe2x80x9d mode.
Wake-up circuits should consume as little power as possible since they are always connected to a (battery) power supply. At the desired point of time, the wake-up circuit receives an external infrared signal which activates the device or a power consuming appliance, respectively, via the switching circuit. In this connection it must also be taken into consideration that in case of an intensive background light, an increased current noise will occur which will deteriorate the signal/noise ratio.
It is an object of the invention to provide a wake-up circuit of the initally defined kind which enables a selective reception and an amplification of the signals also in case of intensive background light, wherein the circuit configuration nevertheless shall be such that an extremely low power consumption is achieved in the waiting (xe2x80x9csleepingxe2x80x9d) state of operation.
According to the invention, this object is achieved in that the input circuit includes a parallel arrangement of at least two series arrangements of infrared detectors as well as a parallel resonant circuit, and in that the amplifier is a bandpass amplifier, the output of which being connected to a signal detector to which the switching circuit is connected. By this configuration, the object set out above can well be met. By the parallel connection of branches containing infrared-detectors in series as well as by the filtering effect of the parallel resonant circuit, an increased signal/noise ratio is attained. The wake-up circuit according to the invention thus is particularly suitable for outdoor use, e.g. for switching on infrared transmittors in toll systems on roads.
According to a preferred embodiment of the invention it is provided that the signal detector is formed with two interconnected integrating circuits and a comparator, the one input of the comparator being connected to the output of the one integrating circuit, and the other input being connected to the output of the other integrating circuit. This measure leads to a selective checking of the duration of the signal, i.e. the switching circuit will only be switched on after a pre-determined minimum duration of the signal.
Here, it is also advantageous if the one integrating circuit comprises a capacitor which is connected to a series arrangement of resistors which form a voltage divider, and if the other integrating circuit comprises a resistor connected to the branch-off point of the voltage divider and a capacitor connected to this resistor, the discharge time constant of the one integrating circuit being shorter than the discharge time constant of the other integrating circuit. Despite a low number of electronic elements, in this manner functioning of the signal detector will be very much ensured irrespective of element tolerances and variations of the supply voltage.
An advantageous and simple embodiment of the bandpass amplifier is characterized in that the bandpass amplifier is a multi-stage transistor amplifier with direct current coupling, with the at-rest working currents of the individual transistors each being adjusted to a minimum, whereby a particularly low load on the supply voltage source will be attained.
To still further improve the selectivity of the wake-up circuit with a view to the frequency of the signals, it is provided that the lower cut-off frequency of the bandpass amplifier is adapted to the lower cut-off frequency of the parallel resonant circuit, resulting in an additional improvement of the signal/noise ratio.